1. Field of the Invention
The present invention relates generally to apparatuses for protecting against overvoltage and, more particularly, to apparatuses for protecting electrical apparatuses, circuits and the like driven by direct current power sources against detrimental overvoltage.
2. Description of the Background Art
A fuse is one representative of conventional apparatuses for protecting electrical apparatuses and circuits against overcurrents applied thereto. The fuse, which is connected in series to a load, is melted by Joule heat generated when an overcurrent higher than a rated current flows through a circuit, and thus is employed to cut energization applied from a power source. In order to achieve this purpose, the fuse has a rated current defined therein, so that the fuse is to be selected in accordance with its own purpose for practical use.
The fuse is less expensive with its simple structure, and thus has been widely used as a representative of safety devices for electrical apparatuses. However, since a fuse can not be reused once it has been melted, there has been a need for a spare fuse.
Meanwhile, other protective apparatuses are proposed such as a circuit breaker and a protector. These proposed apparatuses can be used repetitively if a cause of the overcurrent is removed; however, they are large-scaled with complicated structures due to the requirement of a switch disengaging mechanism which operates in response to the overcurrent, and thus are expensive and require a large space to be installed. Therefore, they cannot be employed as easily as the fuse.
Moreover, the circuit breaker and the fuse both employ energy caused by the overcurrent, and require much time and energy of a certain level or more to achieve their own purposes.
Meanwhile, as in the case of a computer, circuits of low operating voltage and small operating, i.e., decreased power requirements have been developed since the appearance of semiconductor elements. An electronic apparatus employing such a circuit should be protected against overvoltage rather than overcurrent. Therefore, it often occurs, for example, that an overvoltage V.sub.S which has a pulse waveform generally called a spike, which is instantaneously applied at the level exceeding the level of a rated voltage V.sub.CC of a load (e.g., 5V), as shown in FIG. 5, results in an abnormal operation of the load and destruction of the circuit. In such a case, energy generated instantaneously is relatively small, so that a power consumption type protective apparatus, such as the described fuse or the breaker, cannot cope with the applied overvoltage. A Zener diode and a varistor serve as apparatuses for protecting against overvoltage which cope with such a problem. However, in the case of employing the Zener diode or the varistor as the apparatus for protecting the electrical apparatus and circuit against overvoltage, the following disadvantages occur, and thus it is impossible to realize a sufficiently quick response to an applied overvoltage.
FIG. 6 shows one example of an electrical circuit diagram employing a Zener diode as the apparatus for protecting against overvoltage.
As shown in this figure, V.sub.R is a variable voltage, R is a resistor, Z.sub.D is a Zener diode, and L is a load connected in parallel to the Zener diode Z.sub.D. The Zener diode Z.sub.D, which is also called a constant voltage diode, has its both ends supplied with a constant voltage so as to protect the load L against a surge. However, when a voltage equal to or higher than a Zener voltage is applied to the load L by a surge current due to an overvoltage applied by the voltage V.sub.R (for example, an overvoltage V.sub.S shown in FIG. 5), the load L is caused to malfunction.
Furthermore, there is another problem that due to a current flowing in the state that the overvoltage is applied, the Zener diode Z.sub.D has its own heat value increased and thus burns out. In addition, there also arises a problem that since the overvoltage is kept applied to the load L connected in the period that the overvoltage is applied, an apparatus corresponding to the load L causes malfunction and thus a burnout.
Further, in the case that the varistor is substituted for the Zener diode Z.sub.D shown in FIG. 6, the same disadvantage as in the Zener diode is caused with respect to the applied overvoltage. Moreover, in the case of employing the varistor as the apparatus for protecting against overvoltage, it is impossible to form the varistor with other circuit elements as a semiconductor integrated circuit. In detail, a manufacturing method of the varistor is complicated because various kinds of oxides are sintered in a reduction atmosphere in the manufacturing process, so that it is impossible to integrate the varistor on the semiconductor integrated circuit.
Therefore, implementation of a small and inexpensive apparatus for protecting against overvoltage has been desirable, which can also be used as easily as a fuse, Zener diode, the varistor or the like, and has a sufficiently quick response even to an overvoltage applied in such a short period as described above.